| Chemical cues sensed by the main olfactory and the vomeronasal systems have important roles in the regulation of innate behaviors such as mating and aggression. In order to gain insight into the processing of sensory information leading to behavioral changes, I have aimed at identifying neural projections from the main olfactory epithelium (MOE) and the vomeronasal organ (VNO) to higher centers of the brain associated with behavior and endocrine control.; To identify afferent chemosensory inputs modulating reproduction, I have traced the afferents of neurons synthesizing luteinizing hormone releasing hormone (LHRH), a key neuro-hormone of reproduction. Injection of conditional pseudorabies virus into the brain of an LHRH::Cre mouse line led to the identification of entire neuronal networks connected to LHRH neurons. Remarkably, and in contrast to established notions on the nature of LHRH neuronal inputs, I was able to identify major olfactory projections originating from a discrete population of olfactory sensory neurons, but failed to document significant synaptic connectivity with the vomeronasal system. Accordingly, chemosensory modulation of LHRH neuronal activity and mating behavior are dramatically impaired in absence of olfactory function, whereas they appear unaffected in mouse mutants with impaired VNO function. Further visualization of afferents to LHRH neurons across the brain uncovered complex polysynaptic circuits modulating reproduction and fertility.; In order to identify vomeronasal inputs involved in the regulation of reproduction, I took a different approach and attempted to map the projections relaying pheromonal information from the VNO to brain centers involved in the control of behaviors. I used Wheat Germ Agglutinin (WGA), an anterograde tracer, fused to different fluorescent proteins and expressed in different subsets of vomeronasal receptor neurons. The analysis of the mouse line in which WGA is expressed with the EC1 receptor of the V2R family revealed that these neurons project to a small population of clustered neurons in the posterodorsal part of the medial amygdala and periventricular nucleus of the hypothalamus. This restricted projection pattern suggests that information from different receptors is segregated in the amygdala and the hypothalamus, and is consistent with the involvement of different receptor population in distinct behavioral responses. |
